Biology 3/4 AOS 2 Enzymes and Photosynthesis

reactant

A substance that is present at the start of a chemical reaction

Products

the substance that is present at the end of a chemical reaction

Biochemical Pathway

A series of interconnected biochemical reactions that convert a reactant into a product, which then becomes a reactant for the next step.

Enzymes

Proteins that act as catalysts within biochemical pathways to speed up reaction, thus lowering activation energy.

Mesophyll

where the chloroplasts are found

stomata

the little holes that control movement of gas through the leaf

Grana

multiple stacks of thylakoids

Granum

a stack of thylakoids

Thylakoids

flattened sacs containing chlorophyll and other pigments. It is the site of light - dependent reactions

stroma

fluid filled space where the Calvin cycle takes place

how many membranes does a chloroplast have

2

word equation for photosynthesis

Light and Chlorophyll

carbon dioxide + water ——————————> glucose + water + oxygen

Biochemical pathway

made up of a number of smaller reactions

chemical equation of photosynthesis

Light and Chlorophyll

6CO₂ + 12H₂O ————————→ C₆H₁₂O + 6H₂O + 6O₂

Where is chlorophyll?

embedded in the thylakoid membrane

Pigments

a coloured compound in plants that is used to absorb and trap light

Chlorophyll A

the major photosynthetic pigment and is found in all photosynthetic organisms

Where are chlorophyll molecules?

embedded int he membrane structure of grana

Chlorophyll function

absorbs wavelengths of violet to blue and red light and reflect green light

carotenoids

accessory pigments found in all plants and absorb blue and green light and reflect yellow light

accessory pigment

better at absorbing light than chlorophyll A (transfers energy to Chlorophyll A to enhance its effectiveness)

Coenzymes in photosynthesis

No proteins that act as a carrier. (loaded and unloaded)

Unloaded ADP + P_i = ?

ATP

Unloaded NADP+ = ?

NADPH

Where does the light dependent reaction occur?

occurs in the thylakoid membrane of the grana and involves trapping light

where does the calvin cycle (light independent reaction) occur

Occurs in the stroma and energy trapped in the first stage is used to make organic compounds from carbon dioxide and water.

what does a light dependent reaction require?

water and light energy

1st step in light dependent reaction

Sunlight is trapped by chlorophyll a (or other pigments) and light energy is converted to chemical energy.

2nd step in light dependent reaction

3rd step in light dependent reactions

O2 molecules are released and H⁺ ions are gathered by a carrier molecule or acceptor molecule (NADP in this case).  

4th step in light dependent reaction

NADP becomes NADPH and transports H⁺ ions from the grana to the stroma.  

5th step in light dependent reaction

H+ ions and ATP produced in light-dependent reaction are utilised in light-independent reaction.

what does the calvin cycle do to carbon

it involves the reduction of carbon

what is the calvin cycle dependent on

dependent on the previous stage occuring

inputs of calvin cycle

CO₂, NADPH, ATP

Outputs of Calvin Cycle

Glucose ( C₆H₁₂O₆), Water (H₂O), ADP + P_i, NADP+

Inputs of light dependent reaction

Water, ADP + P_i, NADP+

What does carbon reduction require

it requires a supply of carbon dioxide and hydrogen ions

where can carbon dioxide come from? (3)

1. cellular respiration reactions

2. the air

3. the leaf

where does the energy come from to drive the reactions in a light independent reaction?

comes from ATP and loaded carriers (NADPH) which was produced during the light dependent stage

What roles do H⁺ and ATP play in the light-independent reactions of photosynthesis when carbon dioxide is converted into organic compounds?

H+ is the reducing agent and ATP is the source of energy for reducing carbon dioxide to organic compounds such as glucose and other sugars.

Why don’t plants form sugars by simply joining CO₂ molecules together, and what type of process is used instead?

Sugar formation involves a cyclic set of reactions in which intermediate substances are formed.

What happens to carbon dioxide during each turn of the Calvin cycle?

Each turn of the Calvin cycle fixes and reduces one carbon dioxide molecule into an organic intermediate.

How many turns of the Calvin cycle are needed to produce one 6-carbon compound?

6 turns

Why can the Calvin cycle continue repeatedly?

The starting compound is regenerated at the end of each cycle, allowing the process to continue as long as CO₂, ATP, and NADPH are available.

In which types of plants does the Calvin cycle occur?

occurs in C3 plants, which include nearly all trees and most shrubs and herbs.

1. Carbon Fixation

CO₂ combines with RuBP (5C) via Rubisco to form a 6C compound, which splits into two 3-PGA molecules.

2. Reduction

ATP and NADPH convert 3-PGA into G3P; NADPH donates electrons to reduce the molecule.

3. Regeneration of RuBP

Some G3P forms glucose, while the rest uses ATP to regenerate RuBP.

limiting factor

a factor that restricts the rate of something

How does light intensity affect the rate of photosynthesis?

The rate increases with light intensity until there is another limiting factor

What percentage of light hitting a leaf is reflected?

20%

What proportion of absorbed light is converted into chemical energy?

1%

What colour does chlorophyll not absorb

green because green light is reflected

Which colours of light are readily absorbed by chlorophyll?

red, blue, purple

Why is white light also effective for photosynthesis?

White light contains all colours, so chlorophyll can absorb the red, blue, and purple components.

What happens if there is insufficient water?

The rate of photosynthesis decreases

Does increasing water always increase photosynthesis?

only increases up to a point

How do most plants obtain carbon dioxide for photosynthesis?

CO₂ from the air dissolves in extracellular fluid before entering photosynthetic cells.

Do carbon dioxide levels remain constant in the environment?

No, there are local variations in CO₂ levels depending on habitat and time of day.

How do aquatic plants obtain carbon for photosynthesis?

Aquatic plants can use hydrogen carbonate (HCO₃⁻) formed when CO₂ dissolves in water.

Can CO₂ from cellular respiration be used for photosynthesis?

Yes, but it usually provides only a small portion of the total CO₂ required.

How does temperature affect the rate of photosynthesis?

Photosynthesis increases with temperature up to about 20–40 °C, then declines due to photorespiration and reduced enzyme activity.

what is the light compensation point

The light compensation point is the light intensity where CO₂ uptake by photosynthesis equals CO₂ produced by respiration.

What happens to CO₂ output at low light

At low light intensity, photosynthesis is slower than respiration, so plants release CO₂.

what stage of photosynthesis will be limited by light intensity

light dependent photosynthesis

what stage of photosynthesis will be limited by CO₂ availability?

light independent photosynthesis

what happens if a plant gets too hot

Photosythesis is limited by the denaturing of Rubisco enzymes

what makes chlorophyll

magnesium and nitrogen

Photorespiration

when o2 binds to rubisco instead of co2 because o2 is a competitive inhibitor

diffusion